1. Field of Invention
This invention relates to laser fuses for use in integrated circuits and more particularly to laser fuse structures which allow increased packing density in an integrated circuit.
2. Related Art
Laser fuses are in common use in the semiconductor industry as circuit elements for customizing alterations of individual integrated circuits (ICs) for the purpose of repair or reconfiguration. During laser configuration, specific fuses are blown open by a targeting laser beam resulting in a desired pattern of blown and not blown fuses as required by the repair or configuration scheme. Using laser fuse-based circuits, it is possible for defective memory bits to be swapped out of a memory array, for custom functions to turned on or off in an application-specific IC (ASIC), and for serial numbers to be written to individual ICs. Despite their usefulness, laser fuses are typically used sparingly in an IC due to the large layout area cost of each fuse, each fuse taking up the area of many transistors. This large area may be generally attributed to the long wavelength of light employed by lasers, the less controllable optical and positioning subsystems used in commercial lasering systems compared with wafer lithography systems, and the need to space off unrelated circuitry away from the laser fuse to avoid collateral damage during fuse blowing.
FIGS. 1, 2A, and 2B illustrate the large area requirements of conventional laser fuses. FIG. 1 shows the physical layout of an array 10 of conventional laser fuses 100, configured to connect perpendicular lines 110 and 120, as required to implement a laser configurable cross-point junction between orthogonal signal buses. Lines 110 are formed from an upper conductive layer, while lines 120 are formed from a lower conductive layer, these layers being separated by an intermediate insulating layer (not shown). A selection of interconnections between lines 110 and 120 is implemented by either blowing or retaining laser fuses 100.
FIG. 2A shows a detailed view of one portion of array 10 of FIG. 1 containing one fuse 100. Laser fuse 100 has a fuse body 210, which is typically the same width (FIG. 2A) or narrower (FIG. 2B) than connection terminals 220 of laser fuse 100. The term fuse body refers to that portion of the fuse structure which is irradiated by the laser beam and removed during lasering. If a disconnection is desired between line 110 and line 120, a laser is directed at an intended beam blast area 230 overlapping fuse body 210, blowing the fuse to effect the desired disconnection. A relatively high laser energy is required to blow fuse 100, and thus connection terminals 220 must be made long enough to protect connection nodes 240 from thermally conducted heat damage during lasering. This additional length adds to the Y-direction pitch as measured between lines 110. Additionally, since the laser beam typically has a radial Gaussian energy distribution, increasing the beam energy tends to laterally spread the beam blast area. Thus, adjacent lines 120 must be spaced off a greater distance in the X-direction to avoid collateral beam damage which might cut into these lines. As a consequence of the X-direction and Y-direction layout requirements, the packing density of conventional laser fuses 100 on an IC surface is relatively low, as is most evident when trying to lay out structures which use a large number of fuses such as a laser configurable cross-point junction between two wide signal buses. The result is an undesirable lower density and/or larger size IC.
Laser fuse designs must also take into account the heat transfer effects that occur during and immediately after fuse blowing. A principle design objective is that the greatest portion of the laser energy goes into heating the fuse body and the lowest portion go into heating surrounding or underlying structures. This minimizes damage to nearby structures, and also lowers the energy required to blow the fuse, thus allowing use of a less powerful and therefore smaller diameter laser beam. This design objective is ideally satisfied by having the fuse body be maximally thermally isolated from other structures. The glass insulator enclosing the fuse body approximates this requirement as it provides good thermal insulation as well as electrical insulation, but the electrical terminals of the fuse are problematic. The materials typically used to form the electrical terminals (either metals or polysilicon) have a high thermal conductivity when compared with the glass insulation, and thus form an undesirable thermal path for heat to escape the fuse body and cause damage to nearby structures, while making it harder to blow the fuse by sapping thermal energy out of the blast zone.
In conventional laser fuse designs, this heat conduction problem is minimized by implementing long fuse connection terminal nodes 220 between the laser blast area and the fuse connection nodes to reduce the heat transferred to the connection nodes. A number of inventions have further addressed the problem of thermal management. For example, in Lou et al., U.S. Pat. No. 5,729,042, entitled "Raised Fuse Structure For Laser Repair", a pedestal structure is disclosed to improve the thermal flow characteristics, and in Shiozaki et al., U.S. Pat. No. 4,682,204, entitled "Fuse Element For Integrated Circuit Memory Device", a corrugated surface is used under the fuse terminals to increase their effective thermal length, both of which are incorporated by reference in their entirety.
Any technique that can reduce the amount of energy required to blow a fuse is also found useful. A well-known technique is to cover the fuse body with a thin layer of glass so as to form a bomb-vessel enclosure of the fuse body. This results in a more uniform vaporization of the fuse and in lower energy requirements when compared with open-top fuses which may splatter and tend to form connective stringers unless shot with high energy or multiple pulses. For example, in Fischer, U.S. Pat. No. 4,853,758, entitled "Laser-Blown Fuses", a fabrication process is disclosed that reduces the energy required to blow a fuse by controlling the thickness of the overlying thin glass layer, and in Gilmour et al., U.S. Pat. No. 5,760,674, entitled "Fusible Links With Improved Interconnect Structure", a distinct intermediate interconnect level is used to space off the fuse body from the electrical terminals of the fuse, both of which are incorporated by reference in their entirety. Note that the lateral interconnections used by Gilmour et al. provide thermal isolation, but still require significant layout space.
Accordingly, it is desirable to have a laser fuse structure which allows increased packing density of laser fuse elements on a IC surface both through minimized layout dimensions and through improved thermal management techniques.